What is Metamorphism?

• Metamorphism: Process where rocks undergo changes in mineralogy, texture, and chemical composition due to changing physical and chemical conditions.
• Occurs predominantly in the solid state.
• Greek Origin: Meta (change) + Morph (form).

Agents of Metamorphism

• Temperature: Most common agent; promotes recrystallization.
• Pressure: Lithostatic pressure (uniform), or differential stress causing deformation.
• Hydrothermal fluid: Promote chemical reactions and transport of ions.
• Stress: Leads to foliation and lineation.

Difference Between Pressure and Stress

Pressure (Lithostatic Pressure)

• Definition: A uniform force applied in all directions, increasing with depth due to the weight of overlying rocks.
• Effect:
  • Promotes the formation of high-density minerals.
  • Causes recrystallization but does not result in deformation.
  • Rocks remain unchanged in shape under pressure.
• Role in Metamorphism: Influences mineral stability and density without changing the rock’s structure.

Stress (Directed or Differential Stress)

• Definition: A directional force that is stronger in one direction than another, resulting in deformation.
• Effect:
  • Causes deformation of rocks, changing their shape.
  • Leads to the development of structures like foliation and lineation.
  • Minerals reorient along planes of weakness due to stress.
• Role in Metamorphism: Creates metamorphic textures such as foliation and plays a key role in strain (physical deformation).

Source: https://opengeology.org

Types of Metamorphism

• Contact Metamorphism: Heat from nearby magmatic intrusions (e.g., hornfels).
• Regional Metamorphism: Large-scale tectonic processes (mountain building).

Source: https://www.msnucleus.org

Contact Metamorphism

Definition

• Contact metamorphism occurs when rocks are heated by nearby magma or lava intrusions.
• Primarily driven by heat with minor contributions from pressure and fluids.

Key Characteristics

• Localized effect around intrusive igneous bodies.
• Creates a metamorphic aureole or halo of altered rock.
• Dominated by high temperature and low pressure conditions.

Common Rocks and Minerals

Marble

• Texture: Coarse-grained, granular.
• Common minerals: Calcite or dolomite.
• Description: Derived from limestone; reacts with acid.

Source: https://openpress.usask.ca

Quartzite

• Texture: Medium to coarse-grained, interlocking quartz grains.
• Common minerals: Quartz.
• Description: Derived from sandstone; very hard and resistant to weathering.

Source: https://openpress.usask.ca

Hornfels

• Texture: Fine-grained, dense, and hard.
• Common minerals: Variable, often with biotite, feldspar, and pyroxene.
• Description: Formed by contact metamorphism; lacks foliation.

Source: https://openpress.usask.ca

Regional Metamorphism

Definition

• Regional metamorphism occurs over large areas due to tectonic forces, involving:
  • Heat
  • Pressure
  • Fluids
• Associated with mountain-building events and plate tectonics.

Key Characteristics

• Occurs at convergent plate boundaries and in deep crustal zones.
• Produces foliated rocks.
• Pressure plays a significant role, often creating layered or banded textures.

Metamorphic Grade

• Low-Grade Metamorphism: Fine-grained minerals; minimal recrystallization (e.g., chlorite).
• High-Grade Metamorphism: Coarse-grained minerals; extensive recrystallization (e.g., garnet).
• Indicators: Mineral assemblages vary based on temperature and pressure conditions.

Source: https://www.geologyin.com/

Metamorphic Textures

• Foliation: Planar fabric resulting from differential stress (e.g., schist, gneiss).
• Non-Foliated: Granoblastic texture in rocks like marble and quartzite.

Source: https://www.zmescience.com/

Hydrothermal Processes

Definition

• Hydrothermal processes involve the interaction of hot water-rich fluids with rocks.
• Driven by:
  • Heat from magma or deep Earth processes.
  • Circulation of fluids through fractures and pore spaces.

Key Characteristics

• Occurs at:
  • Mid-ocean ridges (e.g., black smokers).
  • Volcanic regions and igneous intrusions.
  • Zones of high fluid flow and permeability.
• Promotes chemical alteration (metasomatism) of rocks.
• Often associated with ore deposits and mineralization.

Example Rocks Formed

• Serpentinite:
  • Formed by hydration of ultramafic rocks (e.g., peridotite).
• Talc:
  • From the alteration of mafic or ultramafic rocks by hydrothermal fluids.

Factors Influencing Hydrothermal Processes

1. Fluid composition: Controls the minerals formed.
2. Temperature: Hotter fluids lead to more intense metamorphism.
3. Host rock type: Determines the resulting mineralogical changes.
4. Permeability: Allows fluids to penetrate and alter rocks.

Summary

• Metamorphism: Changes in mineralogy, texture, and chemistry of rocks in the solid state.
• Agents:
  • Temperature: Drives recrystallization.
  • Pressure: Lithostatic (uniform) or directed (stress).
  • Hydrothermal fluids: Promote chemical changes.
• Types:
  • Contact Metamorphism: Localized heat; forms marble, quartzite, hornfels.
  • Regional Metamorphism: Large-scale tectonic forces; forms schist, gneiss.
• Hydrothermal Processes:
  • Interaction of hot fluids with rocks.
  • Produces rocks like serpentinite and talc.
• Textures:
  • Foliated: Layers (e.g., schist, gneiss).
  • Non-foliated: Granoblastic (e.g., marble, quartzite).
• Metamorphic Grade: Low to high, indicated by mineral assemblages (e.g., chlorite to garnet).